This invention relates generally to gas turbine engines, and more specifically to turbine casings used with gas turbine engines.
At least some known gas turbine engines generally include, in serial flow arrangement, a high pressure compressor for compressing air flowing through the engine, a combustor in which fuel is mixed with the compressed air and ignited to form a high energy gas stream, and a high pressure turbine (HPT). The high pressure compressor, combustor, and high pressure turbine are sometimes collectively referred to as the core engine. Such gas turbine engines also may include a low pressure compressor, or booster, for supplying compressed air to the high pressure compressor.
At least some known turbines include a rotor assembly including a plurality of rows of rotor blades. Each rotor blade extends radially outward from a blade platform to a tip. A plurality of shrouds couple together to form a flow path casing that extends substantially circumferentially around the rotor assembly, such that a tip clearance is defined between each respective rotor blade tip and the HPT casing. Ideally, the tip clearance is designed to be a minimum, while still being sized large enough to facilitate rub-free engine operation through a range of available engine operating conditions.
During operation, turbine performance may be influenced by the running tip clearance between turbine blade tips and the HPT casing. Specifically, as the clearance increases, leakage across the rotor blade tips may adversely limit the performance of the turbine assembly. Maintaining tight running clearances during steady state take-off operation facilitates reducing exhaust gas temperatures (EGT) overshoot. Moreover, maintaining such clearances during cruise operations facilitates reducing specific fuel consumption (SFC).
Accordingly, to facilitate maintaining blade tip clearance at least some known HPT casing designs attempt to substantially match the rate of thermal expansion of the stator case to the rate of thermal expansion of the turbine rotor assembly by controlling the casing temperature. In some instances, thermal matching of the HPT casing may be achieved via a combination of passive elements (casing thickness and thermal mass) and active elements (for example, directing cool air onto the external surface of the casing). Limiting the thermal response of the HPT casing facilitates reducing EGT overshoot and SFC.